Image capture device, method of capturing image with the same, and irradiation device

ABSTRACT

An image capture device  1001  captures an image by using a terahertz wave and includes a generating unit  112  that includes a plurality of generation elements each of which generates the terahertz wave and rests on a resting plane  117,  an irradiation optical system  111  that irradiates an object with the terahertz wave, an imaging optical system  101  that images the terahertz wave that is reflected from the object, and a sensor  102  that includes pixels. The plurality of generation elements include at least a first generation element  113  and a second generation element  114  that have different angles of radiation to the object. There is an overlap region in which a region of radiation of a first terahertz wave  156  from the first generation element to the object overlaps a region of radiation of a second terahertz wave  157  from the second generation element to the object.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Patent ApplicationNo. PCT/JP2017/041330, filed Nov. 16, 2017, which claims the benefit ofJapanese Patent Application No. 2016-230606, filed Nov. 28, 2016, bothof which are hereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to an image capture device that usesterahertz waves, a method of capturing an image with the image capturedevice, and an irradiation device.

BACKGROUND ART

Terahertz waves are electromagnetic waves that typically have componentsin a frequency band from 0.3 THz to 30 THz. In the frequency band, thereare many kinds of characteristic absorption that originate from thestructure and state of various substances, starting with biomoleculesand resins. In addition to this, the wavelength thereof is longer thanthose of visible light and infrared light.

Accordingly, terahertz waves are unlikely to be affected by scatteringand have high permeability against many substances. The wavelength isshorter than those of millimeter waves, and spatial resolution is high.

There are expectations of applications to, for example, a safe imagingtechnique in place of X-rays and a high resolution transmission imagingtechnique in place of millimeter waves (typically, 30 GHz to 300 GHz)and a spectrum imaging technique achieved by making the best use of theabove characteristics. For example, applications to a concealed-objectinspection technique such as a security check or a surveillance camerain public is considered.

PTL 1 discloses that an image capture device uses a terahertz waveirradiation device, beams of terahertz waves from a terahertz wavegeneration element that is considered as a point light source areenlarged and radiated to an object, and the terahertz waves are receivedby detector arrays.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Laid-Open No. 2006-81771

SUMMARY OF INVENTION

An image capture device according to an aspect of the present inventioncaptures an image of an object by using a terahertz wave and includes agenerating unit that includes a plurality of generation elements each ofwhich generates the terahertz wave and rests on a resting plane, anirradiation optical system that images the terahertz wave from thegenerating unit on an imaging plane, an imaging optical system thatimages the terahertz wave that is reflected from the object, and asensor that includes pixels and that detects the terahertz wave from theimaging optical system. The generating unit rests on an object plane ofthe irradiation optical system. The plurality of generation elementsinclude at least a first generation element and a second generationelement that are adjacent to each other in the generating unit and thathave different angles of radiation to the object. There is an overlapregion in which a beam of a first terahertz wave from the firstgeneration element to the object and a beam of a second terahertz wavefrom the second generation element to the object overlap on the imagingplane.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A schematically illustrates the structure of an image capturedevice according to a first embodiment.

FIG. 1B schematically illustrates the structure of the image capturedevice according to the first embodiment.

FIG. 2 schematically illustrates an example of another structure of theimage capture device according to the first embodiment.

FIG. 3 schematically illustrates an example of arrangement of pointlight sources.

FIG. 4 schematically illustrates the structure of an image capturedevice according to a second embodiment.

FIG. 5 schematically illustrates the structure of an image capturedevice according to a third embodiment.

FIG. 6A schematically illustrates the structure of a scanning unit of animage capture device according to a fourth embodiment.

FIG. 6B schematically illustrates the structure of the scanning unit ofthe image capture device according to the fourth embodiment.

FIG. 7 schematically illustrates an example of another structure of thescanning unit of the image capture device according to the fourthembodiment.

FIG. 8 schematically illustrates the structure of a shape adjustmentunit of the image capture device according to the fourth embodiment.

FIG. 9 schematically illustrates an example of another structure of thescanning unit of the image capture device according to the fourthembodiment.

FIG. 10A schematically illustrates an example of calculation of beamdistribution of the image capture device according to the firstembodiment.

FIG. 10B schematically illustrates an example of calculation of the beamdistribution of the image capture device according to the firstembodiment.

FIG. 10C schematically illustrates an example of calculation of the beamdistribution of the image capture device according to the firstembodiment.

FIG. 11 illustrates a relationship between an overlap ratio of the beamdistribution and a distance between the point light sources of the imagecapture device according to the first embodiment.

FIG. 12 is a flowchart illustrating a method of capturing an imageaccording to the fourth embodiment.

FIG. 13 illustrates background noise and atmospheric attenuation ofelectromagnetic waves.

DESCRIPTION OF EMBODIMENTS

The skin structure of the human body has irregularities of several 10 μmto several 100 μm. The wavelength of terahertz waves is in the rangefrom several 10 μm to several 100 μm or more than the range as with theskin structure. For this reason, in the case where an object includesthe human body, imaging with the terahertz waves is not scatteringimaging with scattering of, representatively, visible light but isspecular reflection imaging with specular reflection. More specifically,the skin structure of the human body can be considered as a smoothreflective object against the terahertz waves. The direction of thespecular reflection waves of the terahertz waves is determined by theposition and angle at which the terahertz waves are incident on a curvedsurface of the human body.

For example, in an attempt to image the human body with an image capturedevice that radiates the terahertz waves by using a point light sourcedisclosed in PTL 1, the specular reflection waves of the terahertz wavedo not reach the detector arrays depending on the direction of thespecular reflection waves of the terahertz waves. For this reason, someof pixels of the image capture device can detect the terahertz waves,but the other pixels cannot detect the terahertz waves in some cases. Asthe ratio of the pixels that cannot detect the terahertz wavesincreases, information about the shape of the object decreases, and itis not easy to presume the detailed shape of the object from a capturedimage.

In view of the above problem, it is an object of embodiments describedlater to inhibit the number of pixels that can detect terahertz wavesfrom decreasing in an image capture device that uses the terahertzwaves.

According to each embodiment described later, the image capture devicethat uses the terahertz waves can inhibit the number of the pixels thatcan detect the terahertz waves from decreasing.

According to each embodiment described later, an irradiation device thatradiates the terahertz waves and the image capture device that uses theirradiation device will be described. The terahertz waves will now bedescribed.

FIG. 13 illustrates an example of background noise (radiant fluxdensity) that originates from the sun and comes to the earth in amicrowave band to a terahertz wave band, and an example of a frequencyspectrum of the amount of atmospheric attenuation in the terahertz waveband. A noise at an increased part of the background noise that isobserved in the microwave band to a millimeter wave band variesdepending on the state of the activity of the sun. As illustrated inFIG. 13, in some cases, the background noise increases in the microwaveband to the millimeter wave band. In the microwave band to themillimeter wave band, an artificial noise due to the activity of a humanbeing and various noises due to the state of the weather and theatmosphere are superposed as environment noises.

In recent years, communication technology mainly in the millimeter waveband and astronomical observation with electromagnetic waves in themillimeter wave band have become popular, and the radio law divides afrequency band of less than 0.275 GHz into fine sections for purposes.The electric field strength that can be outputted in the millimeter waveband is strictly restricted by the radio law because this band is alsoused for the astronomical observation.

In the case where an image capture device that uses the millimeter wavesis constructed, a frequency conversion technique that uses a multiplierand a signal that has a small SN ratio is frequently used for detectionbecause of increase in the background noise and restriction of theoutputs of the millimeter waves that can be used. In addition, thewavelength of the electromagnetic waves that are used is long, the sizeof an optical system that includes an image sensor increases, and thereis a concern that the size of the image capture device increaseselectrically and optically. For some use, a sufficient SN ratio cannotensured, and it is necessary for a pixel size of the image sensor to beincreased. Accordingly, an image that is captured is limited to theentire contour of the object, and it is difficult to directly identifythe detailed shape of the object in some cases.

The use of the image capture device that uses the terahertz waves can beconsidered to make an image of a millimeter wave camera more precise.For example, it can be expected that the image capture device that usesthe terahertz waves can use a light source that has a higher output thanthat in the case where the millimeter waves are used, there are manychoices of usable frequencies, and the device size can be decreasedbecause the wavelength is decreased.

As seen from the spectrum of the amount of atmospheric attenuation inthe terahertz wave band in FIG. 13, there is a region (referred to as an“atmospheric window”) in which the atmospheric attenuation is small.Accordingly, it can be considered that selecting the electromagneticwave related to the atmospheric window enables great signal attenuationto be prevented from occurring.

According to the embodiments described later, the image capture devicethat uses the terahertz waves, a method of capturing an image, and theirradiation device that is used in the image capture device will bedescribed. The object of each embodiment described later is to inhibitthe number of the pixels that can detect the terahertz waves fromdecreasing even in the image capture device that uses the terahertzwaves for specular reflection imaging as described above. An imagecapture device that uses the millimeter waves potentially has the sameproblem. However, the problem more notably surfaces when the shape ofthe object is imaged with high precision by using the image capturedevice that uses the terahertz waves and that achieves higher resolutionthan the image capture device that uses the millimeter waves and thatimages the entire contour of the object.

First Embodiment

An image capture device 1001 according to the present embodiment will bedescribed with reference to FIG. 1. FIG. 1 schematically illustrate thestructure of the image capture device 1001.

The image capture device 1001 includes a detection unit 100, a firstirradiation device (first irradiation unit) 110, a second irradiationdevice (second irradiation unit) 120, a first support unit 118, a secondsupport unit 119, a monitor unit 130, and a processing unit 170.

The first irradiation unit 110 and the second irradiation unit 120irradiate an object 140 with the terahertz waves. According to thepresent embodiment, the image capture device 1001 includes twoirradiation units (irradiation devices) of the first irradiation unit110 and the second irradiation unit 120. However, the number of theirradiation units is not limited thereto and may be 1 or 2 or more. Theterahertz waves that are generated from the first irradiation unit 110are radiated to the object 140 as first irradiation waves 153. Theterahertz waves that are generated from the second irradiation unit 120are radiated to the object 140 as second irradiation waves 154.

The frequency of the terahertz waves from the first irradiation waves153 and the second irradiation waves 154 preferably includes a componentin a frequency band or a single frequency in the range from no less than0.3 THz and no more than 30 THz in which the frequency is not assigned.In the case where the object 140 includes the human body, many clotheshave high permeability up to 1 THz. Accordingly, in the case of, forexample, a concealed-object inspection, a frequency range of no lessthan 0.3 THz and no more than 1 THz is more preferable.

The first irradiation unit 110 and the second irradiation unit 120 eachinclude at least a generating unit 112 that generates the terahertzwaves and an irradiation optical system 111. The first irradiation unit110 will be described later. The second irradiation unit 120 has thesame structure.

The generating unit 112 includes generation elements that include afirst generation element 113 and a second generation element 114 thatgenerate the terahertz waves, and corresponds to a surface light sourcethat rests on a resting plane 117.

The size of each generation element is less than the distance to thedetection unit 100, and the generation element can be considered as apoint terahertz wave source and is referred to below as a point lightsource. In other words, the generation element is a terahertz wavesource the size of which is substantially the same as a size that can beresolved as an image by the detection unit 100 or is smaller than thesize. In this case, it can be considered that the point light sourcegenerates a terahertz wave radially from a single point. The restingplane 117 will be described later. In the following description, each ofthe generation elements that include the first generation element 113and the second generation element 114 is referred to as the “point lightsource”, and the generating unit that includes the generation elementsis referred to as the “surface light source”.

Examples of each point light source can include a terahertz wavegeneration element of a semiconductor element such as aresonant-tunneling diode, and a photoexcitation terahertz wavegeneration element that uses optical switching and difference frequencylight.

Each point light source preferably has an antenna structure to improveimpedance matching with the air and the efficiency of generation of theterahertz waves. The size of an antenna is determined to besubstantially equal to the wavelength that is used.

The first point light source 113 and the second point light source 114that are included in the point light sources will be described below byway of example. The first point light source 113 generates a firstterahertz wave 156. The second point light source 114 generates a secondterahertz wave 157. There is an overlap region in which a region ofradiation of the first terahertz wave 156 partly overlaps a region ofradiation of the second terahertz wave 157.

In this case, the distance between the first point light source 113 andthe second point light source 114 is preferably equal to or longer thana distance that is obtained from the longest wavelength of thewavelengths of the first terahertz wave 156 and the second terahertzwave 157. Specifically, the distance between the first point lightsource 113 and the second point light source 114 is equal to or morethan a far field of each antenna that corresponds to the longestwavelength of the wavelengths of the first terahertz wave 156 and thesecond terahertz wave 157. The wavelength of the first terahertz wave156 and the wavelength of the second terahertz wave 157 may be the sameor may differ from each other.

The “far field” in the specification means a distance at which the pointlight sources 113 and 114 are considered to be separated from eachother. The far field is expressed in various manners. For example, thefar field is a distance of 2D2/λ or more where D is the diameter of eachantenna, and λ is the wavelength of the terahertz waves. The distancebetween the point light sources is more preferably about 32D2/λ, whichis considered as infinity. In a state where the second point lightsource 114 is disposed at the far field of the first point light source113, the point light sources can be considered as independent lightsources, and mutual effects between the point light sources can beignored, which results in stable operation.

For example, in the case where half-wavelength antennas (D=λ/2) such asdipole antennas or patch antennas are used as the antennas of the pointlight sources 113 and 114, the far field can be calculated to be 0.5λ ormore. In particular, the distance that is considered as infinity can becalculated to be 8λ or more. In the case where the first terahertz wave156 and the second terahertz wave 157 are terahertz waves at 0.5 THz(λ=0.6 mm), the far field is 0.3 mm, the distance that is considered asinfinity is 4.8 mm. In the case where the terahertz waves that are usedhave plural wavelengths, λ is the longest wavelength.

The irradiation optical system 111 irradiates the object with theterahertz waves. The irradiation optical system 111 according to thepresent embodiment has an imaging function. Specifically, the firstirradiation waves 153 that are generated from the surface light source112 that rests on an object plane 116 of the irradiation optical system111 are converged on an imaging plane 115 of the irradiation opticalsystem 111. The object plane 116 is the imaging plane of the irradiationoptical system 111 facing the object. The first irradiation waves 153are combination waves of the terahertz waves that include at least thefirst terahertz wave 156 and the second terahertz wave 157. The numberof the terahertz waves that are included in the combination waves isequal to the number of the point light sources that are included in thesurface light source 112.

The irradiation optical system 111 may include a transmissive opticalelement such as a lens or a reflective optical element such as a mirror,or a combination thereof. For example, in the image capture device 1001in FIG. 1, the irradiation optical system 111 the optical axis of whichcoincides with a straight line 150 includes a single lens. In the casewhere the lens is used as the irradiation optical system 111, thematerial of the lens preferably has a small loss against the terahertzwaves that are used. Examples thereof include Teflon (registeredtrademark) and high density polyethylene. A method for visible light canbe used for the design of the irradiation optical system 111.

The structure of the irradiation optical system 111 is not limited tothat of a transmissive optical system. For example, as illustrated inFIG. 2, a reflective irradiation optical system 211 that uses a mirrormay be used as the irradiation optical system 111. The irradiationoptical system 211 of an image capture device 1002 in FIG. 2 uses themirror that reflects the terahertz waves from the point light sources,and the mirror has an off-axis paraboloid shape the optical axes ofwhich coincide with straight lines 250. However, the structure of themirror is not limited thereto.

In the image capture device 1002, a surface light source 212 includesthe point light sources that rest on a resting plane 217 that intersectswith an object plane 216 to adjust to the structure of the irradiationoptical system 211. First irradiation waves 253 that include a firstterahertz wave 256 from the first point light source 113 and a secondterahertz wave 257 from the second point light source 114 via theirradiation optical system 211 are imaged on an imaging plane 215 andradiated to the object 140. A second irradiation unit 220 has the samestructure. Second irradiation waves 254 from the second irradiation unit220 are radiated to the object 140.

The use of the transmissive optical element illustrated in FIG. 1 as theirradiation optical system 111 enables the surface light source 112 andthe irradiation optical system 111 to be coaxially arranged. For thisreason, when the irradiation units 110 and 120 are constructed, theaccuracy of alignment can be readily ensured. The coaxial arrangementenables an installation space to be decreased and enables the size ofthe irradiation units 110 and 120 to be decreased.

The use of the reflective optical element illustrated in FIG. 2 as theirradiation optical system 111 enables a loss when the terahertz wavespass through the optical element to be reduced and inhibits the outputsof the first irradiation waves 153 and the second irradiation waves 154from decreasing. The size of a reflective optical system is easy toincrease more than in the case of a transmissive optical system.Accordingly, the terahertz-wave-receiving area of the irradiationoptical system 111 can be increased, and the efficiency of reception ofthe terahertz waves can be improved.

The detection unit 100 is a terahertz wave camera that detects theterahertz waves. In the image capture device 1001, the first irradiationunit 110 and the second irradiation unit 120 are secured to thedetection unit 100 by using the first support unit 118 and the secondsupport unit 119 and integrally formed. Each of the first support unit118 and the second support unit 119 may include a posture adjustmentmovable portion that adjusts the postures of the first irradiation unit110 and the second irradiation unit 120.

The detection unit 100 includes a sensor 102 that includes dividedpixels and an imaging optical system 101 that images reflected waves155, which are terahertz waves, from the object 140 on an imaging planeof the sensor 102. The reflected waves 155 include the second terahertzwave 157 and the first terahertz wave 156 that are reflected from theobject 140.

The pixels of the sensor 102 are divided into an array shape or a matrixshape. The pixels include respective detection elements that detect theterahertz waves. Examples of each detection element can include athermal detection element such as a bolometer or a semiconductordetection element such as a Schottky barrier diode. A terahertz waveimage is formed with reference to an output signal of the sensor 102.

Each detection element of the sensor 102 preferably has an antennastructure to improve impedance matching with the air and the efficiencyof detection of the terahertz waves. The size of each antenna isdetermined to be substantially equal to the wavelength that is used inthe image capture device 1001. In the case where it is necessary tocapture the image quickly, the semiconductor detection element ispreferably used as the detection element.

The imaging optical system 101 images, on the sensor 102, an image ofthe object 140 that is on the object plane of the imaging optical system101, and an optical element such as a lens or a mirror can be used. Eachimage capture device 1 uses a single lens the optical axis of whichcoincides with a straight line 151 as the imaging optical system 101.However, the structure of the imaging optical system 101 is not limitedthereto, and plural optical elements may be used. In the case of thelens, a material that has a small loss against the terahertz waves thatare used is preferably used. For example, Teflon and high-densitypolyethylene can be used. A method for visible light can be used for thedesign of the imaging optical system 101.

The reflected waves 155 from the object 140 are detected by thedetection unit 100. The detection result of the detection unit 100 issent to the processing unit 170. The processing unit 170 captures animage by using the detection result of the detection unit 100. Examplesof the processing unit 170 can include a processing apparatus such as acomputer that includes, for example, a CPU (central processing unit), amemory, and a storage device. A process for visualization may beperformed by software in the processing unit 170. Some functions thatare achieved by processes of the processing unit 170 can be substitutedby hardware such as a logic circuit. The processing unit 170 may be ageneral-purpose computer or exclusive hardware such as a board computeror an ASIC. Alternatively, the processing unit 170 may be installed inthe detection unit 100.

The monitor unit 130 can display the image of the object on the basis ofinformation about the image that is formed by the processing unit 170.The monitor unit 130 may be a monitor of a computer that serves as theprocessing unit 170 or may be prepared to display the image.

FIG. 1B schematically illustrates a part of the imaging plane 115 of theirradiation optical system 111. The imaging plane 115 has an overlapregion in which a part of a first beam distribution (first radiationregion) 158 of the first terahertz wave 156 that is converged on theimaging plane 115 overlaps a part of a second beam distribution (firstradiation region) 159 of the second terahertz wave 157 that is convergedon the imaging plane 115. In the surface light source 112, the distancebetween the first point light source 113 and the second point lightsource 114 and the arrangement thereof are preferably adjusted such thatthe part of the first beam distribution 158 overlaps the part of thesecond beam distribution 159.

With this structure, a region of the object 140 is irradiated with thefirst terahertz wave 156 and the second terahertz wave 157 in differentdirections. Consequently, the first terahertz wave 156 and the secondterahertz wave 157 are reflected from the object 140 at reflectionangles that are equal to incident angles and travel in differentdirections from the object 140. This enables the first terahertz wave156 and the second terahertz wave 157 that are reflected from the regionof the object 140 to be considered as pseudo scattering waves.

At this time, the overlap region on the imaging plane 115 between thefirst beam distribution 158 and the second beam distribution 159preferably overlaps an observation region 160 that corresponds to atleast one of the pixels of the sensor 102 on the imaging plane 115.

In this way, each pixel of the sensor 102 of the detection unit 100 canreceive specular reflection light in different directions from thecorresponding observation region 160, and the percentage of the pixelsthat cannot detect the terahertz waves can be decreased. Consequently,the image can be captured by using the detection result of the detectionunit 100 more accurately than in a conventional case. In addition, theshape of the object 140 can be presumed from the captured image moreeasily than in the conventional case.

The point light sources rest on the resting plane 117. The resting plane117 may be a flat surface or may contain a curved surface. The restingplane 117 may be flush with the object plane 116 of the irradiationoptical system 111 or may intersect therewith. The first irradiationunit 110 adjusts the shape of the resting plane 117 and the posture ofthe resting plane 117 with respect to the object plane 116 to adjust thedistance between each point light source and the irradiation opticalsystem 111, and adjusts aberration of the terahertz waves that areradiated to the object 140. The adjustment of the aberration of theterahertz waves enables the overlap region between the beamdistributions of the terahertz waves from the point light sources to beadjusted and enables the degree of overlap with the observation region160 to be adjusted.

Since the size of the irradiation optical system 111 is definite, thereis a possibility that so-called vignetting occurs, that is, some of theterahertz waves are eliminated by the optical element of the irradiationoptical system 111 depending on the shape of the resting plane 117 andthe posture of the resting plane 117 with respect to the object plane116. For example, the vignetting can decrease the outputs of theterahertz waves that reach the object 140. In order to reduce thevignetting, as illustrated in

FIG. 3, a single point on the optical axis of an irradiation opticalsystem 311 preferably intersects with a single point on a directionalaxis of a beam pattern (radiation pattern) of the terahertz wave that isradiated from each of the point light sources that include point lightsources 313 and 314.

The directional axis of each point light source in the specificationmeans the central axis of directional characteristics of the terahertzwave from the point light sources. Specifically, the directional axiscoincides with a straight line that represents the direction in whichthe terahertz wave that has the maximum strength is emitted from thepoint light source. For example, the directional axis coincides with astraight line that connects positions at which the strength of theterahertz wave is maximum on concentric circles that have differentradii and that have the center located at the center of gravity of thepoint light source.

For example, as illustrated in FIG. 3, a directional axis 361 of aradiation pattern 360 of the point light source 313 that is included ina surface light source 312, a second directional axis 363 of a secondradiation pattern 362 of the point light source 314 that is included inthe surface light source 312, and the optical axis of the irradiationoptical system 311 intersect with each other at the same position. Sucharrangement enables the terahertz waves that are generated from thepoint light sources to be contained in an optically effective region ofthe irradiation optical system 311. Consequently, the vignetting that iscaused by the irradiation optical system 311 is reduced, and the outputsof the terahertz waves that reach the object 140 can be inhibited fromdecreasing.

According to the present embodiment, the position at which thedirectional axis 361 intersects with the optical axis of the irradiationoptical system 311 is the same as the position at which the directionalaxis 363 intersects with the optical axis of the irradiation opticalsystem 311. However, the present embodiment is not limited to thisstructure. That is, the directional axes 361 and 363 may intersect withthe optical axis of the irradiation optical system 311 at differentpositions.

The terahertz waves from the point light sources are preferably radiatedto the object 140 at the same time. In the case where the outputs of thefirst point light source 113 and the second point light source 114 aremodulated, the point light sources preferably change the outputs to theobject 140 synchronously.

FIG. 10B and FIG. 10C illustrate examples of geometrical opticscalculation of the beam patterns of the terahertz waves that are imagedon the imaging plane 215 of the image capture device 1002 in FIG. 2 fromthe surface light source 212. Specifically, rays are tracked from thepoint light sources that are included in the surface light source 212 tothe imaging plane 215.

FIG. 10A illustrates the arrangement of the point light sources that areincluded in the surface light source 212 that is used for thecalculation. The surface light source 212 is used for the calculation onassumption that the surface light source 212 includes point lightsources [1] to [9] that are arranged at a central portion of the surfacelight source 212 and that are arranged d spaced apart from each other,and point light sources [10] to [17] that are arranged on an outercircumferential portion of the surface light source 212. The point lightsources [1] to [9] at the central portion are used to see overlapbetween the beam distributions of the terahertz waves. The point lightsources [10] to [17] on the outer circumferential portion are used tosee the maximum expansion of the beam distributions due to theaberration.

For simplicity of the calculation, here, the surface light source 212includes the nine point light sources [1] to [9] that are arranged atthe central portion and the eight point light sources [10] to [17] thatare arranged on the outer circumferential portion. However, the numberand position of the point light sources are not limited thereto. Forexample, in the case where the point light sources are distance d spacedapart from each other and arranged in a matrix shape, the number of thepoint light sources of the surface light source 212 may be(L/d+1)×(L/d+1) where L is the length of an outer circumferential sideof the surface light source 212.

The conditions for the calculation will be described. In FIG. 10A, thelength L of the side of the surface light source 212 is 100 mm. In thecase where the target frequency is 0.5 THz, the wavelength λ of theterahertz waves is 0.6 mm. A half-wavelength antenna is used as theantenna of each point light source. The diameter D of the antenna is 0.3mm. In this case, the far field of the antenna is 0.3 mm (λ/2) or more.The far field that is considered as infinity is 4.8 mm (8λ) or more.

A typical parabolic antenna for satellite broadcasting is used as theirradiation optical system 211. The length of an aperture of theparabolic antenna in the longitudinal direction is 520 mm and the lengththereof in the transverse direction is 460 mm. The depth from theaperture to the bottom is 50 mm. The on-axis focal length of theparabolic antenna is 234 mm, the off-axis angle thereof is 55.6 degrees,and the off-axis focal length thereof is 299 mm.

The off-axis focus of the parabolic antenna that serves as theirradiation optical system 211 is located on the incidence axis 250 ofthe terahertz waves that reach the irradiation optical system 211 fromthe surface light source 212. The inclination of the aperture of theirradiation optical system 211 is 62.2 degrees with respect to theincidence axis 250. The incidence axis 250 corresponds to ageometrically optical axis. The object plane 216 is perpendicular to theincidence axis 250. The object plane 216 passes through a point on theincidence axis 250. Specifically, the object plane 216 is located at aposition about 85 mm from the off-axis focus in the direction away fromthe irradiation optical system 211. The surface light source 212 isdisposed near the object plane 216 such that the object plane 216 andthe resting plane 217 intersect with each other. The resting plane 217may match the object plane 216.

In the case where such a first irradiation unit 210 is used, the firstirradiation waves 253 are imaged on a location about 1340 mm away fromthe irradiation optical system 211, and the terahertz waves are radiatedto the object 140. When the length L of the outer circumferential sideof the surface light source 212 is 100 mm as described above, the firstirradiation waves 253 that are radiated to the object 140 have adimension of about 350 mm×350 mm. In the calculation, the effectivediameter of the aperture of the parabolic antenna is 80%. The secondirradiation unit 220 has the same structure as the first irradiationunit 210.

FIG. 10B illustrates calculation of the beam distributions, on theimaging plane 215, of the terahertz waves from the point light sources[1] to [9] at the central portion of the surface light source 212 whenthe distance d between the point light sources is 4.8 mm (8λ), which isconsidered as infinity of each antenna. The horizontal axis(Horizontal/mm) corresponds to the X-direction in FIG. 2, and thevertical axis (Vertical/mm) corresponds to the Y-direction in FIG. 2.

As illustrated in FIG. 10B, the beam distributions of the terahertzwaves extend so as to protrude upward because of an effect of theaberration of the parabolic antenna. It can be seen that the beams ofthe terahertz waves from the point light sources [1] to [4] and [6] to[9] around the point light source [5] overlap the beams of the terahertzwaves from the point light source [5] that is disposed at the center.Consequently, the reflected waves 155 can be used as pseudo scatteringwaves. The reflected waves 155, which are the pseudo scattering wavesthat are reflected at the overlap portion, can be detected in a mannerin which the observation region 160 of each pixel of the sensor 102 ofthe detection unit 100 is caused to overlap the overlap portion.

It is here assumed that a lens that has an outer diameter of 120 mm anda curvature of about 100 mm is used as the imaging optical system 101and the distance between the sensor 102 and the imaging optical system101 is 224 mm. In this case, the distance between the imaging opticalsystem 101 and the object 140 is about 1200 mm and can be substantiallyequal to the distance between the first irradiation unit 210 and theobject 140. When the pixel size of the sensor 102 is 0.5 mm, which issubstantially equal to the wavelength of the surface light source 212,the size of the observation region 160 is about 2.6 mm. In FIG. 10B, thesize of the overlap region (region illustrated in a circle in FIG. 10B)of the beam distributions is larger than the size of the observationregion 160. For this reason, it can be understood that the overlapregion can contain the observation region 160. The shape of the imagingoptical system 101 may include an aspherical surface.

FIG. 10C illustrates the result of the calculation of the beamdistributions of the terahertz waves from the point light sources [1] to[17] when the distance d between the point light sources is 19.2 mm(32λ). The number in the figure is the number of each point light sourcethat is illustrated in FIG. 10A and that is used for the calculation. Asillustrated in FIG. 10C, the beams of the terahertz waves from the pointlight source [5] that is disposed at the center overlap the beams of theterahertz waves from the point light sources [1] to [4] and [6] to [9]around the point light source [5] at two positions. Specifically, thebeams of the terahertz waves of the point light source [5] overlap thebeams from the terahertz waves from the point light sources [4] and [6].

The beam distributions of the terahertz waves from the point lightsources [10] to [17] on the outer circumferential portion of the surfacelight source 212 are larger than the beam distributions of the pointlight sources [1] to [9] at the central portion due to the effect of theaberration of the irradiation optical system 211. For this reason, thenumber of the point light sources the beam distributions of whichoverlap the observation region 160 can be increased.

An overlap ratio of the terahertz waves from the adjacent point lightsources will now be described. The “overlap ratio” in the specificationis a ratio B/A of the number B of regions that overlap the beam at thecentral portion to the number A of the beams of the terahertz waves fromthe point light sources that are adjacent to the point light source thatis disposed at the center, and is a ratio of the adjacent beamdistributions that overlap. In the case in FIG. 10B, the distance dbetween the point light sources is 4.8 mm (8λ), and the overlap ratiois 1. In the case in FIG. 10C, the distance d between the point lightsources is 19.2 mm (32λ), and the overlap ratio is 0.25.

In FIG. 11, the overlap ratio between the adjacent beams is plotted forthe distance d of the adjacent point light sources at the centralportion of the surface light source 212. As illustrated in FIG. 11, allof the beams of the terahertz waves from the adjacent point lightsources overlap within the far field 8 X that is considered as infinity,and the overlap ratio of the beams decreases outside 8λ. The beamsscarcely overlap outside 32λ. The beam distributions are isolated fromeach other at 36λ.

It can be understood from above that the distance between the firstpoint light source 113 and the second point light source 114 that ispreferable to form the pseudo scattering waves at the terahertz waveband can be defined by the value of the far field that is defined by thewavelength X. Specifically, as illustrated in FIG. 10C, the distance dbetween the first point light source 113 and the second point lightsource 114 is preferably no less than 0.5λ and no more than 36λ in orderto form the pseudo scattering waves in the wavelength range of theterahertz waves. The distance d between the first point light source 113and the second point light source 114 is more preferably no less than0.5λ and no more than 8λ. The structure of the irradiation units 210 and220 and the detection unit 100 is not limited to the above structure andis appropriately designed in accordance with components that are used inthe image capture device and the shape of the object 140 to be observed.

With this structure, the pixels of the sensor of the camera can receivethe specular reflection light in different directions by radiating theterahertz waves to the object in different directions. This enables theterahertz waves that are reflected from the observation region to beconsidered as the pseudo scattering waves. For this reason, thepercentage of the pixels that cannot detect the terahertz waves can bedecreased. Consequently, the resolution of the image that is captured byusing the terahertz waves is improved, and the shape of the object canbe readily presumed.

Second Embodiment

An image capture device 1003 according to the present embodiment will bedescribed with reference to FIG. 4. FIG. 4 schematically illustrates thestructure of the image capture device 1003. The image capture device1003 differs from the image capture device 1002 according to the firstembodiment in that irradiation units 410 and 420 are arranged in adifferent manner. Common components to those according to the aboveembodiment are designated by like reference characters in FIG. 4, and adetailed description thereof is omitted.

In the image capture devices 1001 and 1002 according to the firstembodiment, the irradiation units 110, 120, 210, and 220 and thedetection unit 100 are combined together by the first support unit 118and the second support unit 119. However, in the image capture device1003 according to the present embodiment, the first irradiation unit 410is held by a first support unit 418 and disposed separately from thedetection unit 100. The second irradiation unit 420 is held by a secondsupport unit 419 and disposed separately from the detection unit 100.The first support unit 418 and the second support unit 419 may hold thepostures of the first irradiation unit 410 and the second irradiationunit 420 and may have a posture adjustment mechanism for adjusting theposture.

The image capture device according to the present embodiment, which usesthe terahertz waves, can inhibit the number of the pixels that candetect the terahertz waves from decreasing.

With the structure of the image capture device 1003 according to thepresent embodiment, the degree of freedom of the arrangement of thefirst irradiation unit 410 and the second irradiation unit 420 isimproved, and the image capture device can be used for a wider range ofapplications.

Third Embodiment

The structure of an image capture device 1004 according to the presentembodiment will be described with reference to FIG. 5. FIG. 5schematically illustrates the structure of the image capture device1004. The positional relationship between an irradiation unit 510 andthe detection unit 100 of the image capture device 1004 differs fromthat according to the above embodiment. Common components to thoseaccording to the above embodiments are designated by like referencecharacters in FIG. 5, and a detailed description thereof is omitted.

Specifically, the irradiation unit 510 of the image capture device 1004is disposed behind the detection unit 100. In other words, the imagingoptical system 101 and an irradiation optical system 511 face each otherwith a surface light source 512 interposed therebetween, and theirradiation unit 510 is disposed such that the axis substantiallycoincides with the optical axis of the detection unit 100.

In this case, the irradiation optical system 511 of the irradiation unit510 is preferably reflective, and the size of an optically effectiveregion of the irradiation optical system 511 is preferably sufficientlylarger than the size of a section of the detection unit 100. Theirradiation unit 510 and the detection unit 100 may be integrally formedor may separate from each other. With this structure, the size of theimage capture device 1004 can be decreased.

Also, according to the present embodiment, a first terahertz wave 556and a second terahertz wave 557 from the point light sources of theirradiation unit 510 overlap on the imaging plane of the irradiationoptical system 511. This enables the image capture device according tothe present embodiment, which uses the terahertz waves, to inhibit thenumber of the pixels that can detect the terahertz waves fromdecreasing.

Fourth Embodiment

An image capture device 1005 according to the present embodiment will bedescribed with reference to FIG. 6. FIG. 6 schematically illustrates thestructure of the image capture device 1005. The image capture device1005 is an equivalent to one of the image capture devices according tothe above embodiments that further includes a structure for scanning theterahertz waves. An example described here is the image capture device1002 according to the first embodiment that further includes thestructure for scanning the terahertz waves. Common components to thoseaccording to the above embodiments are designated by like referencenumbers in FIG. 6, and a detailed description is omitted.

The image capture device 1002 according to the first embodiment includesno mechanisms for scanning the terahertz waves, and the direction inwhich the terahertz waves are radiated to the object 140 is almostfixed, or a posture control unit, not illustrated, controls the posturesof the irradiation units 210 and 220 to change the direction of theirradiation waves. The present embodiment, however, further includes ascanning unit 690 that scans the irradiation waves by simultaneouslychanging the postures of the first irradiation unit 210, the secondirradiation unit 220, and the detection unit 100. Consequently, theincident angles and radiation ranges of the terahertz waves to theobject 140 can be changed. Since the incident angles of the terahertzwaves that are incident on the observation region 160 of each pixel ofthe sensor 102 can be changed, the reflection angles of the terahertzwaves from the observation region 160 are also changed.

Examples of the scanning unit 690 can include an angle adjustment stagethat adjusts an angle of elevation and an angle of direction (rotationangle) and a linear motion stage that adjusts the position of the imagecapture device 1002. According to the present embodiment, a rotationstage that adjusts the angle of elevation is used as the scanning unit690. As illustrated in FIG. 6A and

FIG. 6B, the positions and angles of radiation of the first irradiationwaves 253 and the second irradiation waves 254 to the object 140 can bechanged by simultaneously adjusting the postures of the firstirradiation unit 210, the second irradiation unit 220, and the detectionunit 100 by the scanning unit 690.

Here, the state in FIG. 6A is referred to as a first state, and thestate in FIG. 6B is referred to as a second state. The image capturedevice 1005 obtains the detection result of the detection unit 100 inthe first state and the detection result of the detection unit 100 inthe second state and captures images from the respective detectionresults. The images are combined with each other. Consequently,reflection angle components of the terahertz waves that travel from thepoint light sources and that are reflected from the observation region160 can be increased, and a state closer to scattering light can beobtained.

The structure of the scanning unit 690 is not limited thereto. Forexample, as illustrated in FIG. 7, posture change units 790 of the firstsupport unit 118 and the second support unit 119 can be used as thescanning unit 690. The posture change units 790 are mechanisms forchanging the postures of the first irradiation unit 210 and the secondirradiation unit 220. The posture change units 790 changes and adjuststhe angles of elevation of the first irradiation unit 210 and the secondirradiation unit 220 to scan the first irradiation waves 253 and thesecond irradiation waves 254 in scanning directions 791. The posturechange unit 790 can be installed in the first support unit 418 or thesecond support unit 419 of the image capture device 1003 according tothe second embodiment, or the posture change units 790 can be installedin both of the first support unit 418 and the second support unit 419.

A method of capturing an image by using the image capture device 1005according to the present embodiment will be described with reference toFIG. 12. The incident angles of the irradiation waves on the object 140are adjusted by the scanning unit 690 and include at least a firstincident angle and a second incident angle. The number of the incidentangles can be set by a measurer as needed or may be determined inadvance in accordance with a measurement mode. In the followingdescription, attention is paid to the first irradiation waves 253.However, the same processes may be performed for the other irradiationwaves.

When measurement is started, the scanning unit 690 adjusts the posturessuch that the incident angles of the irradiation waves 253 on the object140 are equal to the first incident angle (S1201). In this state, theirradiation waves 253 are radiated to the object 140, and the detectionresult that is obtained by detecting the reflected waves 155 from theobject 140 by the detection unit 100 is used to capture a first image(S1202). First image data D1201 is stored in a storage unit of theprocessing unit. Subsequently, the scanning unit 690 adjusts thepostures such that the incident angles of the irradiation waves 253 onthe object 140 are equal to the second incident angle (S1203).

In this state, the irradiation waves 253 are radiated to the object 140,and the detection result that is obtained by detecting the reflectedwaves 155 from the object 140 by the detection unit 100 is used tocapture a second image (S1204). Second image data D1202 is stored in astorage unit of the processing unit. This process is repeated the samenumber of times as the number of the incident angles that are set tocapture the images.

Subsequently, the processing unit reads the first image data D1201 andthe second image data D1202 that are stored in the storage unit, notillustrated, and combines the images (S1205). In this manner, thereflection angle components of the terahertz waves that are reflectedfrom the observation region 160 can be increased, and a state closer toscattering light can be obtained. Consequently, the percentage of thepixels that cannot detect the terahertz waves can be decreased.Consequently, an image that has higher resolution than that in theconventional case can be captured, and the shape of the object can bereadily presumed from the captured image. The monitor unit 130 candisplay the combined image.

The method of capturing an image described according to the presentembodiment is an example, and the order of each step can be changed.Multiple steps can be performed at the same time. The step of capturingthe image such as the step S1202 may be omitted, and information aboutthe image that is captured at the step S1204 may be obtained from thedetection results of the detection unit 100 that are obtained withdifferent postures.

In the case of the image capture devices described above, the firstirradiation waves (153, 253) that include the first terahertz wave (156,256) and the second terahertz wave (157, 257) are imaged on the object140 in a circular plane as illustrated in FIG. 1B. However, the firstirradiation waves 153 and 253 are not limited thereto and can beconverged linearly. For example, as illustrated in FIG. 8, a shapeadjustment unit 854 can be disposed on the optical axis (incidence axis250) of the irradiation optical system 211 between the irradiationoptical system 211 and the object 140, and the first irradiation waves253 can be irradiation waves 853 that have linear beam distribution.

The shape adjustment unit 854 can be an optical element in which thecurvature of the axis of one of the irradiation optical system 211 andthe imaging optical system 101 differs from the curvature of the axis ofthe other optical system that is perpendicular to the axis. Examplesthereof can include a cylindrical lens or a cylindrical mirror. In FIG.8, a cylindrical lens through which the terahertz waves pass is used asthe shape adjustment unit 854. The shape of the irradiation waves thatis adjusted by the shape adjustment unit 854 is not limited to a linearshape and may be a circular shape or a quadrilateral shape.

The beam shape of the first irradiation waves 253, which are theterahertz waves, is thus concentrated to irradiate the object 140therewith. This enables the outputs of the terahertz waves that areradiated to the observation region 160 can be increased. Consequently,the SN ratio of the terahertz waves that are obtained by an imagecapture device 1006 is improved, and the gradation of the image of theobject that is captured by using the terahertz waves is improved.

In the case where the image capture device includes the above shapeadjustment unit 854, as illustrated in FIG. 9, a scanning unit 990 thatcontrols the posture of the shape adjustment unit 854 may be included.For example, in FIG. 9, the scanning unit 990 can scan the irradiationwaves 853 in a scanning direction 991 by adjusting the angle ofelevation of the shape adjustment unit 854.

With this structure, the reflection angle components of the terahertzwaves that are radiated from the point light sources and that arereflected from the observation region 160 of the object 140 can beincreased, and a state closer to scattering light can be obtained. Thisenables the image capture device according to the present embodiment,which uses the terahertz waves, to inhibit the number of the pixels thatcan detect the terahertz waves from decreasing. Consequently, thepercentage of the pixels that cannot detect the terahertz waves can bedecreased, and the shape of the object can be readily presumed from theobtained terahertz wave image.

Preferred embodiments of the present invention are described above. Thepresent invention, however, is not limited to the embodiments, andvarious modifications and alterations can be made within the range ofthe spirit thereof. The structures of the image capture devicesaccording to the above embodiments can be combined for use. Accordingly,a new image capture device may be obtained by appropriately combiningvarious techniques according to the above embodiments. The image capturedevice that is obtained by the combination is also included in the scopeof the present invention.

The present invention is not limited to the above embodiments. Variousmodifications and alterations can be made without departing from thespirit and scope of the present invention. Accordingly, the followingclaims are attached to make the scope of the present invention public.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

1. An image capture device that captures an image by using a terahertzwave, the image capture device comprising: a generating unit thatincludes a plurality of generation elements each of which generates theterahertz wave and rests on a resting plane; an irradiation opticalsystem that irradiates an object with the terahertz wave from thegenerating unit; an imaging optical system that images the terahertzwave that is reflected from the object; and a sensor that includespixels and that detects the terahertz wave from the imaging opticalsystem, wherein the plurality of generation elements include at least afirst generation element and a second generation element that havedifferent angles of radiation to the object, and wherein there is anoverlap region in which a region of radiation of a first terahertz wavefrom the first generation element to the object overlaps a region ofradiation of a second terahertz wave from the second generation elementto the object.
 2. The image capture device according to claim 1, whereinthe overlap region overlaps an observation region that corresponds to atleast one of the pixels on an imaging plane of the irradiation opticalsystem.
 3. The image capture device according to claim 1, wherein atleast a part of the generating unit rests on an object plane of theirradiation optical system.
 4. The image capture device according toclaim 1, wherein the resting plane and the object plane intersect witheach other.
 5. The image capture device according to claim 4, whereinthe resting plane includes a curved surface.
 6. The image capture deviceaccording to claim 1, wherein the first generation element rests suchthat a first directional axis of a radiation pattern of the firstterahertz wave intersects with an optical axis of the irradiationoptical system, and wherein the second generation element rests suchthat a second directional axis of a radiation pattern of the secondterahertz wave intersects with the optical axis of the irradiationoptical system.
 7. The image capture device according to claim 1,wherein the irradiation optical system and the imaging optical systemare coaxial with each other with the generating unit interposedtherebetween.
 8. The image capture device according to claim 1, whereina distance between two adjacent generation elements is no less than 0.5λand no more than 36λ, where λ is a longest wavelength of wavelengths ofterahertz waves from the two adjacent generation elements of theplurality of generation elements.
 9. The image capture device accordingto claim 8, wherein the distance between the two adjacent generationelements is no less than 0.5λ and no more than 8λ.
 10. The image capturedevice according to claim 1, wherein the first generation element andthe second generation element are adjacent to each other.
 11. The imagecapture device according to claim 1, further comprising: a scanning unitthat changes an incident angle of the terahertz wave that is incidentfrom the generating unit on the object from a first incident angle intoa second incident angle; and a processing unit that captures an image ofthe object by using a detection result of a detection unit at the firstincident angle and a detection result of the detection unit at thesecond incident angle.
 12. The image capture device according to claim1, wherein the irradiation optical system includes a transmissiveoptical element.
 13. The image capture device according to claim 1,wherein the irradiation optical system includes a reflective opticalelement.
 14. The image capture device according to claim 1, wherein theterahertz wave from the generating unit includes a terahertz wave at noless than 0.3 THz and no more than 30 THz.
 15. The image capture deviceaccording to claim 1, wherein the terahertz wave from the generatingunit includes a terahertz wave at no less than 0.3 THz and no more than1 THz.
 16. An irradiation device comprising: a generating unit thatincludes a plurality of generation elements each of which generates aterahertz wave and rests on a resting plane; and an irradiation opticalsystem that irradiates an object with the terahertz wave from thegenerating unit, wherein the plurality of generation elements include atleast a first generation element and a second generation element thathave different angles of radiation to the object, and wherein there isan overlap region in which a region of radiation of a first terahertzwave from the first generation element to the object overlaps a regionof radiation of a second terahertz wave from the second generationelement to the object.
 17. A method of capturing an image, the methodcomprising: a first generation step of generating a first terahertz wavefrom a first generation element; a second generation step of generatinga second terahertz wave from a second generation element; a convergencestep of converging the first terahertz wave and the second terahertzwave on an object; an imaging step of imaging the first terahertz waveand the second terahertz wave that are reflected from the object; and adetection step of detecting the first terahertz wave and the secondterahertz wave that are imaged at the imaging step, wherein the firstterahertz wave and the second terahertz wave overlap on the object.